High energy storage properties of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics by incorporating Sr0.8Bi0.1□0.1Ti0.8Zr0.2O2.95 
Microstructures 2023;3:2023023. 10.20517/microstructures.2023.04 |
© The Author(s) 2023. Author Information Article Notes Cite This Article Abstract
Ceramics with high-energy storage density are in high demand across various industries. In this regard, lead-free relaxor ferroelectric ceramics were synthesized using the conventional solid-state reaction method with the composition (1-x)[0.94Bi0.5Na0.5TiO3-0.06BaTiO3]-xSr0.8Biγ0.1Ti0.8Zr0.2O0.95, abbreviated as BNBT-xSBTZ. The incorporation of SBTZ in BNBT ceramics significantly improved their relaxation properties. Specifically, the 0.91BNBT-0.09SBTZ ceramics displayed a breakdown electric field of up to 230 kV/cm, with a recoverable energy storage density (Wr) of 2.68 J/cm3 and an energy storage efficiency (η) of 74.4%. Additionally, this sample demonstrated remarkable temperature stability and fatigue resistance, with only an 11% decrease in Wr observed from room temperature to 140 °C and a 13.3% reduction in Wr after 105 electrical cycles. Therefore, the 0.91BNBT-0.09SBTZ ceramic is a promising dielectric material suitable for energy-storage dielectric capacitors.
Keywords
Energy storage properties, lead-free ceramics, temperature stability, fatigue resistance INTRODUCTION
The global community is facing the challenge of climate change and environmental issues, which necessitates the exploration of alternative energy sources such as solar and wind power. However, the intermittent nature of these energy sources presents limitations on their practical applications. Therefore, it is crucial to develop new energy storage technologies. Currently, the primary electrical energy storage devices include batteries, chemical capacitors, and ceramic capacitors. Ceramic capacitors, in particular, offer many advantages, such as high-power density, rapid charging and discharging rates, and a broad operating temperature range. As a result, they are widely used in various applications, including pulsed power supplies, microwave circuits, and electric vehicles. Nevertheless, the low energy storage density of ceramic capacitors compared to chemical batteries restricts their potential applications. Consequently, there is an urgent need to develop ceramic capacitors with higher energy storage density to address the growing energy challenges.
The key performance indicators of ceramic capacitors comprise the recoverable energy storage density (Wr), total energy storage density (W), and energy storage efficiency (η). These parameters can be derived from the electrical hysteresis loops (P-E) of ceramics or calculated by following Equation (1-3)[1,2]:
$$ \begin{equation} \begin{aligned} W = \int_{0}^{P_{m}}EdP \end{aligned} \end{equation} $$ $$ \begin{equation} \begin{aligned} W_{r} = \int_{P_{r}}^{P_{m}}EdP \end{aligned} \end{equation} $$ $$ \begin{equation} \begin{aligned} \eta = \frac{W_{r}}{W} \end{aligned} \end{equation} $$ where Pm is the maximum polarization, and Pr is the remanent polarization.
Equations (1-3) indicate that achieving high energy storage capacity (Wr) and efficiency (η) requires a large polarization (Pm) and a small remnant polarization (Pr). The ferroelectric ceramic Na0.5Bi0.5TiO3 (BNT) possesses a large Pm, making it a promising candidate for developing high-energy storage capacitors. However, pristine BNT ceramic has a large Pr, resulting in low Wr and η. Recent studies have focused on developing BNT-based ceramics with large Pm and small Pr through doping, which can significantly reduce the grain size of the ceramics and increase their dielectric breakdown strength (DBS), Wr, and η[3]. For instance, Qi et al. prepared linear-like anti-ferroelectric BNT-based ceramics with a Wr of 7.02 J/cm3 and η of 85%[4]. Similar results have been reported in other studies[5-8]. Therefore, chemical doping of pure BNT is an important method for preparing ceramics with high-energy storage properties.
The material 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 (BNT-BT) is a type of relaxor ferroelectric material that exhibits a morphotropic phase boundary (MPB) structure. When an external electric field is applied, this material undergoes a reversible transition from a relaxor state to a long-range ferroelectric order, resulting in a large Pm and a lower Pr[9,10]. However, the dielectric breakdown electric field of BNT-BT is relatively low. In general, the breakdown electric field of dielectric ceramics increases with decreasing grain size. Therefore, in this study, we aim to improve the breakdown electric field and enhance the energy storage properties of BNT-BT by using Sr0.8Bi0.1γ0.1Ti0.8Zr0.2O2.95 (SBTZ) as a modifier to regulate the relaxation behavior and decrease the grain size. SBTZ is a newly developed relaxor ferroelectric material obtained by doping Bi3+ and Zr4+ into the cubic-phase perovskite material of SrTiO3. The introduction of Bi3+ and Zr4+ ions into the A-site of SrTiO3 reduces the chemical ratio of Bi3+ and creates an A-site vacancy, which disrupts the ferroelectric order and results in relaxation behavior[11,12]. It is expected that the energy storage performance of BNT-BT will be improved due to the enhanced relaxation property and breakdown electric field.
MATERIALS AND METHODS
Ceramic samples of (1-x)[0.94Bi0.5Na0.5TiO3-0.06BaTiO3]-xSr0.8Biγ0.1Ti0.8Zr0.2O0.95 (abbreviated as: (1-x)BNBT-xSBTZ, x = 0.07, 0.08, 0.09, and 0.10) were synthesized using the solid-state reaction method. The starting materials used were Bi2O3 (99.9%), Na2CO3 (99.5%), BaCO3 (99.9%), SrCO3 (99%), TiO2 (99.6%), and ZrO2 (99.99%). The raw powders were mixed and ball-milled for 16 h in polyethylene pots with zirconia balls and ethanol as the milling medium. The use of ethanol was preferred due to its low ion dissolution properties, which minimized the introduction of unwanted ions. The milled powder was dried, sieved, and formed into cylindrical pellets before being sintered for 4 h at 850 °C The sintered pellets were then crushed, re-milled for 10 h, and cold isostatically pressed into pellets with a diameter of 10 mm under a pressure of 250 MPa. The pellets were sintered at 1150 °C for 2 h in alumina crucibles in an air atmosphere. To compensate for the loss of sodium oxides and bismuth, the pellets were buried in prepared powders of the same composition. Finally, the sintered pellets were polished to a final thickness of approximately 0.6 mm, and silver electrodes were coated on both surfaces and fired at 550 °C for 30 min for electrical testing.
The phase structure was determined through the use of X-ray diffractometry (XRD, Advanced), while the surface morphology was analyzed by means of scanning electron microscopy (SEM) (Carle Zeiss GeminiSEM 500, Germany). In addition, the dielectric properties were measured using a precision impedance analyzer (Agilent E4980A), while Raman spectra were obtained using a Raman spectrometer (Horiba Allmentation LabRam). Finally, the polarization hysteresis loops (P-E) were assessed via a standard ferroelectric analyzer (TF Analyzer 2000).
RESULTS AND DISCUSSION
The X-ray diffraction patterns of BNBT-xSBTZ ceramics were analyzed in the range of 20° to 75°, as shown in Figure 1A. The results indicate that each sample has a single perovskite structure, with no presence of any secondary phase. This suggests that SBTZ is fully incorporated into the lattice[13]. Furthermore, Figure 1B displays the expanded XRD patterns around the diffraction peak of (111), which reveals that the diffraction peaks shift towards higher diffraction angles as the SBTZ doping content increases. This shift indicates a reduction in the lattice constant[14,15]. Additionally, the absence of peak splitting at around 40° suggests that all samples possess a highly symmetrical pseudocubic structure[16].
Figure 1. XRD patterns and Raman spectra of the sintered ceramics. (A) XRD patterns from 20° to 75°; (B) expanded XRD patterns at around peak (111); (C) Raman spectra in the range of 100-900 cm-1.
The Raman spectra of (1-x)BNBT-xSBTZ ceramics were analyzed in the range of 100-1,000 cm-1, as depicted in Figure 1C. The spectra were categorized into four regions, each representing distinct chemical bonding behavior[17]. The modes below 200 cm-1 were found to be associated with A-site atomic vibrations, while the high-frequency band between 200 and 380 cm-1 corresponded to the vibrations of BO bonds, including Zr2+, Zr4+, and Ti4+ ions. The modes near 550 cm-1 and 610 cm-1 were linked to Ti/Zr-O6 vibrations, and it was observed that the two peak positions became separated, indicating an increased intensity of oxygen octahedron vibration and enhanced relaxation characteristics with increasing doping content[18]. The section with modes above 700 cm-1 was considered to be the superposition of various vibration modes.
The SEM images of (1-x)BNBT-xSBTZ ceramics, as depicted in Figure 2A-D, indicate that the ceramics have been effectively sintered with minimal surface pores. The grain size of the ceramics initially decreases and then increases with increasing dopant concentration. This phenomenon can be attributed to the appearance of low melting point regions in the ceramics after SBTZ doping. During the sintering and cooling process, these regions are more prone to nucleation and grain growth, resulting in a decrease in grain size. However, at higher doping concentrations, the liquid phase melting regions further increase, leading to the emergence of smaller grain sizes[19]. The breakdown field of ceramics is significantly influenced by the grain size, with smaller grain sizes resulting in higher breakdown electric fields[18]. Therefore, the addition of SBTZ at low doping levels is advantageous for achieving high energy storage properties. Specifically, when the SBTZ content is 0.07, 0.08, 0.09, and 0.1, the average grain sizes are 1.47 μm, 0.93 μm, 0.90 μm, and 1.22 μm, respectively.
Figure 2. The surface morphology of the (1-x)BNBT-xSBTZ ceramics and distribution of grain size. (A) x = 0.07, (B) x = 0.09, (C) x = 0.09, and (D) x = 0.10.
Figure 3 depicts the dielectric behavior of (1-x)BNBT-xSBTZ ceramics over a temperature range of 35-400 °C and at frequencies of 1 kHz, 10 kHz, 1 MHz, 10 MHz, and 20 MHz, respectively. The curves illustrate that the ceramics exhibit relaxation behavior with double dielectric peaks. The shape of the curves remains consistent across varying SBTZ doping levels, with the dielectric peak Tm becoming broader, indicating an enhanced relaxation characteristic[20]. The second peak Tm, which corresponds to the maximum dielectric constant (εm), represents the relaxation of the rhombohedral-tetragonal phase transition brought about by the polar nanoregions[21]. As the doping level increases, the εm of the ceramics tends to decrease, indicating a weakening of the ferroelectric properties. The low dielectric losses (tan δ) of the samples demonstrate their excellent insulating properties, which play a crucial role in improving the energy storage performance of ceramics.
Figure 3. Temperature-dependent dielectric permittivity and loss of (1-x)BNBT-xSBTZ ceramics from 70 to 400 °C at 1 kHz, 10 kHz, 1 MHz, 10 MHz, and 20 MHz, respectively. (A) x = 0.07, (B) x = 0.09, (C) x = 0.09, and (D) x = 0.10.
In order to conduct a more thorough investigation into the relaxor transition in ceramics, an adaptation of the Curie-Weiss Law was implemented, which can be expressed as follows:
$$ \begin{equation} \begin{aligned} \ln \left(1 / \varepsilon-1 / \varepsilon_{\max }\right)=\gamma \ln \left(T-T_{\mathrm{m}}\right)+\mathrm{C} \end{aligned} \end{equation} $$ where ε is the dielectric constant, T is the temperature, Tm is the temperature at which ε reaches its maximum value εmax, C is a constant, and is the diffuse degree. The value of γ ranges from 1 for typical ferroelectrics to 2 for ideal relaxor ferroelectrics.
Figure 4 depicts the relationship between ln(T-Tm) and ln(1/ε-1/εmax) for the examined samples. The calculated value of γ ranges from 1.76 to 2.01, indicating a strong relaxation behavior of the sintered ceramics.
Figure 4. Plots of ln(1/ε-1/εmax) as a function of ln(T-Tm) at 1 kHz by the modified Curie-Weiss law.
The dielectric breakdown strength (DBS) of the ceramics was evaluated, as depicted in Figure 5A. By utilizing the Weibull distribution as Equation (5-7)[22]:
Figure 5. Energy storage studies of sintered (1-x)BNBT-xSBTZ ceramics. (A) Weibull distribution of the breakdown electric field; (B) P-E loops as a function of x; (C) Pm, Pr, and Pm-Pr as a function of x; (D) Wr, W, and η as a function of x.
$$ \begin{equation} \begin{aligned} P_{\mathrm{i}}=\mathrm{i}(\mathrm{n}+1) \end{aligned} \end{equation} $$ $$ \begin{equation} \begin{aligned} X_{\mathrm{i}}=\ln \left(E_{\mathrm{i}}\right) \end{aligned} \end{equation} $$ $$ \begin{equation} \begin{aligned} Y_{\mathrm{i}}=\ln \left(\ln \left(1 /\left(1-P_{\mathrm{i}}\right)\right)\right) \end{aligned} \end{equation} $$ where i is the group number, Xi and Yi are parameters in Weibull distribution functions, Ei is the DBS of sample i, Pi is associated with dielectric breakdown, and n is the sum of specimens of each sample. The electric field is ranged as Equation (8).
$$ \begin{equation} \begin{aligned} E_{1} \leq E_{2} \leq \ldots \leq E_{\mathrm{i}} \leq \ldots \leq E_{\mathrm{n}} \end{aligned} \end{equation} $$ According to the Weibull distribution model, there is a linear relationship between Xi and Yi, as shown in Equation (9).
$$ \begin{equation} \begin{aligned} Y_{\mathrm{i}}=\mathrm{m} X_{\mathrm{i}}+\mathrm{b} \end{aligned} \end{equation} $$ where m represents the dispersion degree of all experimental data, and a larger m indicates more reliable experimental results.
The determination of the magnitude of DBS can be derived from the intercept on the ln(Ei) axis. The results obtained from the fitted calculations in Figure 5A indicate that the DBS gradually increases and then decreases with the increase of SBTZ content, which is consistent with the shift in the average grain size. The DBS values obtained for increasing dopant concentrations were 215 kV/cm, 230 kV/cm, 245 kV/cm, and 218 kV/cm, respectively. The DBS is influenced by several factors, including porosity, grain size, and extrinsic measurement conditions such as sample thickness, sample area, and electrode configuration. In this study, all the samples had almost the same phase structure and measurement condition, but there was a slight difference in grain size. Generally, smaller grain size contributes to an enhanced DBS, and furthermore, the DBS exhibits an opposite variation tendency with grain size[23-25].
The polarization hysteresis (P-E) loops of the sintered BNBT-xSBTZ ceramics were measured and shown in Figure 5B. The test electric field was loaded near the breakdown field. The P-E loops were observed to be flat and slender, indicating weak ferroelectricity and enhanced relaxation properties. This is a crucial factor in achieving high energy storage density and efficiency[26]. As the doping content increased, Pmax, Pr, and Pmax-Pr gradually decreased, as depicted in Figure 5C. The trend of decreasing polarization intensity also suggests a decline in ferroelectricity. The sample with 0.93BNBT-0.07SBTZ exhibited the highest Pmax of 44.38 μC/cm2, but its Wr was not the highest due to its high Pr. In contrast, the 0.9BNBT-0.1SBTZ sample had a lower Pmax of 29.04 μC/cm2 and Pr of 2.36 μC/cm2. Figure 5D illustrates the values of W, Wr, and η for all samples. It was observed that Wr reached its maximum value at x=0.09, while η was highest at x=0.10. The values of W, Wr, and η are presented in Table 1.
Table 1Energy storage properties of (1-x)BNBT-xSBTZ ceramics
x W (J/cm3) Wr (J/cm3) E (kV/cm) η 0 1.38 0.174 60 12.6% 0.07 3.02 1.91 215 63.1% 0.08 3.21 2.37 230 73.9% 0.09 3.60 2.68 245 74.4% 0.10 2.84 2.38 218 83.9%
The practical application of energy storage ceramics requires good temperature stability. To investigate the temperature stability of the 0.91BNBT-0.09SBTZ ceramic, energy storage characteristics were examined at various temperatures. P-E hysteresis loops were measured at 170 kV/cm, with temperatures ranging from 25 °C to 140 °C, as depicted in Figure 6A. The P-E loops did not exhibit significant changes, with Pmax decreasing from 34.7 μC/cm2 at 25 °C to 31.6 μC/cm2 at 140 °C. Additionally, Wr decreased from 1.71 J/cm3 to 1.52 J/cm3, as shown in Figure 6B. Despite the temperature variations, Wr and η remained at a relatively high level, indicating excellent temperature stability.
Figure 6. Temperature stability and fatigue resistance of the sintered ceramics. (A) P-E hysteresis loops at 25 °C to 140 °C; (B) Wr and η as a function of temperature; (C) fatigue behavior measured at various electrical cycles; (D) Wr and η as a function of electrical cycles.
In addition to examining the temperature stability of ceramics, their fatigue resistance behavior was also investigated. The fatigue behavior was assessed at room temperature under a frequency of 10 Hz and a voltage of 170 kV/cm, with a cycling number of 105. The P-E loops at different cycles were depicted in Figure 6C, while the changes in Pmax and Wr with an increase in the number of charging/discharging cycles were illustrated in Figure 6D. After 105 cycles, Wr remained at a relatively high level of 1.5 J/cm3, which was only 13.3% lower than its initial value of 1.73 J/cm3, indicating favorable fatigue resistance properties. It is worth noting that Pmax and Wr exhibited a slight decrease, which was attributed to the local phase decomposition caused by switching-induced charge injection, as suggested by Lou et al.[27-29].
CONCLUSIONS
This study investigated the dielectric and energy storage properties of (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xSr0.8Bi0.1γ0.1Ti0.8Zr0.2O2.95 ceramics, prepared through the solid-state method. X-ray diffraction analysis revealed a single perovskite structure in all ceramic samples, indicating successful incorporation of SBTZ into the BNBT lattice. Doping with SBTZ resulted in notable relaxation properties and excellent energy storage capabilities. Specifically, 0.91BNBT-0.09SBTZ ceramics demonstrated a breakdown electric field of 230 kV/cm, with Wr of 2.68 J/cm3 and η of 74.4%. Additionally, the energy storage properties of the ceramics exhibited remarkable temperature stability and fatigue resistance. The energy storage density only decreased by 11% from room temperature to 140 °C, and after 105 electrical cycles, the energy storage density decreased by only 5.2%. Overall, these findings suggest that 0.91BNBT-0.09SBTZ ceramics have great potential as dielectric materials for energy-storage dielectric capacitors.
DECLARATIONS
Authors’ contributionsMade substantial contributions to the conception and design of the study and performed data analysis and interpretation: Wang C
Performed data acquisition and provided administrative, technical, and material support: Lou X
Availability of data and materialsNot applicable.
Financial support and sponsorshipThis work was supported by the Education Science and Technology Innovation Project of Gansu Province, China, the year of 2022; the project: Study on the mechanism of the fatigue behavior of sodium-bismuth-titanate based relaxor ferroelectric capacitors after electrical charge-discharge cycles (Grant No. 2022QB-213) and the National Natural Science Foundation of China (NSFC NO.52172125).
Financial support and sponsorshipNone.
Conflicts of interestBoth authors declared that there are no conflicts of interest.
Ethical approval and consent to participateNot applicable.
Consent for publicationNot applicable.
Copyright© The Author(s) 2023.
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Wang C, Lou X. High energy storage properties of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics by incorporating Sr0.8Bi0.1□0.1Ti0.8Zr0.2O2.95Microstructures 2023;3:2023023. http://dx.doi.org/10.20517/microstructures.2023.04
AMA Style
Wang C, Lou X. High energy storage properties of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics by incorporating Sr0.8Bi0.1□0.1Ti0.8Zr0.2O2.95Microstructures. 2023; 3(3): 2023023. http://dx.doi.org/10.20517/microstructures.2023.04
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Wang, Cheng, Xiaojie Lou. 2023. "High energy storage properties of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics by incorporating Sr0.8Bi0.1□0.1Ti0.8Zr0.2O2.95Microstructures. 3, no.3: 2023023. http://dx.doi.org/10.20517/microstructures.2023.04
ACS Style
Wang, C.; Lou X. High energy storage properties of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics by incorporating Sr0.8Bi0.1□0.1Ti0.8Zr0.2O2.95Microstructures. 2023, 3, 2023023. http://dx.doi.org/10.20517/microstructures.2023.04
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Abstract
Ceramics with high-energy storage density are in high demand across various industries. In this regard, lead-free relaxor ferroelectric ceramics were synthesized using the conventional solid-state reaction method with the composition (1-x)[0.94Bi0.5Na0.5TiO3-0.06BaTiO3]-xSr0.8Biγ0.1Ti0.8Zr0.2O0.95, abbreviated as BNBT-xSBTZ. The incorporation of SBTZ in BNBT ceramics significantly improved their relaxation properties. Specifically, the 0.91BNBT-0.09SBTZ ceramics displayed a breakdown electric field of up to 230 kV/cm, with a recoverable energy storage density (Wr) of 2.68 J/cm3 and an energy storage efficiency (η) of 74.4%. Additionally, this sample demonstrated remarkable temperature stability and fatigue resistance, with only an 11% decrease in Wr observed from room temperature to 140 °C and a 13.3% reduction in Wr after 105 electrical cycles. Therefore, the 0.91BNBT-0.09SBTZ ceramic is a promising dielectric material suitable for energy-storage dielectric capacitors.
Keywords
INTRODUCTION
The global community is facing the challenge of climate change and environmental issues, which necessitates the exploration of alternative energy sources such as solar and wind power. However, the intermittent nature of these energy sources presents limitations on their practical applications. Therefore, it is crucial to develop new energy storage technologies. Currently, the primary electrical energy storage devices include batteries, chemical capacitors, and ceramic capacitors. Ceramic capacitors, in particular, offer many advantages, such as high-power density, rapid charging and discharging rates, and a broad operating temperature range. As a result, they are widely used in various applications, including pulsed power supplies, microwave circuits, and electric vehicles. Nevertheless, the low energy storage density of ceramic capacitors compared to chemical batteries restricts their potential applications. Consequently, there is an urgent need to develop ceramic capacitors with higher energy storage density to address the growing energy challenges.
The key performance indicators of ceramic capacitors comprise the recoverable energy storage density (Wr), total energy storage density (W), and energy storage efficiency (η). These parameters can be derived from the electrical hysteresis loops (P-E) of ceramics or calculated by following Equation (1-3)[1,2]:
where Pm is the maximum polarization, and Pr is the remanent polarization.
Equations (1-3) indicate that achieving high energy storage capacity (Wr) and efficiency (η) requires a large polarization (Pm) and a small remnant polarization (Pr). The ferroelectric ceramic Na0.5Bi0.5TiO3 (BNT) possesses a large Pm, making it a promising candidate for developing high-energy storage capacitors. However, pristine BNT ceramic has a large Pr, resulting in low Wr and η. Recent studies have focused on developing BNT-based ceramics with large Pm and small Pr through doping, which can significantly reduce the grain size of the ceramics and increase their dielectric breakdown strength (DBS), Wr, and η[3]. For instance, Qi et al. prepared linear-like anti-ferroelectric BNT-based ceramics with a Wr of 7.02 J/cm3 and η of 85%[4]. Similar results have been reported in other studies[5-8]. Therefore, chemical doping of pure BNT is an important method for preparing ceramics with high-energy storage properties.
The material 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 (BNT-BT) is a type of relaxor ferroelectric material that exhibits a morphotropic phase boundary (MPB) structure. When an external electric field is applied, this material undergoes a reversible transition from a relaxor state to a long-range ferroelectric order, resulting in a large Pm and a lower Pr[9,10]. However, the dielectric breakdown electric field of BNT-BT is relatively low. In general, the breakdown electric field of dielectric ceramics increases with decreasing grain size. Therefore, in this study, we aim to improve the breakdown electric field and enhance the energy storage properties of BNT-BT by using Sr0.8Bi0.1γ0.1Ti0.8Zr0.2O2.95 (SBTZ) as a modifier to regulate the relaxation behavior and decrease the grain size. SBTZ is a newly developed relaxor ferroelectric material obtained by doping Bi3+ and Zr4+ into the cubic-phase perovskite material of SrTiO3. The introduction of Bi3+ and Zr4+ ions into the A-site of SrTiO3 reduces the chemical ratio of Bi3+ and creates an A-site vacancy, which disrupts the ferroelectric order and results in relaxation behavior[11,12]. It is expected that the energy storage performance of BNT-BT will be improved due to the enhanced relaxation property and breakdown electric field.
MATERIALS AND METHODS
Ceramic samples of (1-x)[0.94Bi0.5Na0.5TiO3-0.06BaTiO3]-xSr0.8Biγ0.1Ti0.8Zr0.2O0.95 (abbreviated as: (1-x)BNBT-xSBTZ, x = 0.07, 0.08, 0.09, and 0.10) were synthesized using the solid-state reaction method. The starting materials used were Bi2O3 (99.9%), Na2CO3 (99.5%), BaCO3 (99.9%), SrCO3 (99%), TiO2 (99.6%), and ZrO2 (99.99%). The raw powders were mixed and ball-milled for 16 h in polyethylene pots with zirconia balls and ethanol as the milling medium. The use of ethanol was preferred due to its low ion dissolution properties, which minimized the introduction of unwanted ions. The milled powder was dried, sieved, and formed into cylindrical pellets before being sintered for 4 h at 850 °C The sintered pellets were then crushed, re-milled for 10 h, and cold isostatically pressed into pellets with a diameter of 10 mm under a pressure of 250 MPa. The pellets were sintered at 1150 °C for 2 h in alumina crucibles in an air atmosphere. To compensate for the loss of sodium oxides and bismuth, the pellets were buried in prepared powders of the same composition. Finally, the sintered pellets were polished to a final thickness of approximately 0.6 mm, and silver electrodes were coated on both surfaces and fired at 550 °C for 30 min for electrical testing.
The phase structure was determined through the use of X-ray diffractometry (XRD, Advanced), while the surface morphology was analyzed by means of scanning electron microscopy (SEM) (Carle Zeiss GeminiSEM 500, Germany). In addition, the dielectric properties were measured using a precision impedance analyzer (Agilent E4980A), while Raman spectra were obtained using a Raman spectrometer (Horiba Allmentation LabRam). Finally, the polarization hysteresis loops (P-E) were assessed via a standard ferroelectric analyzer (TF Analyzer 2000).
RESULTS AND DISCUSSION
The X-ray diffraction patterns of BNBT-xSBTZ ceramics were analyzed in the range of 20° to 75°, as shown in Figure 1A. The results indicate that each sample has a single perovskite structure, with no presence of any secondary phase. This suggests that SBTZ is fully incorporated into the lattice[13]. Furthermore, Figure 1B displays the expanded XRD patterns around the diffraction peak of (111), which reveals that the diffraction peaks shift towards higher diffraction angles as the SBTZ doping content increases. This shift indicates a reduction in the lattice constant[14,15]. Additionally, the absence of peak splitting at around 40° suggests that all samples possess a highly symmetrical pseudocubic structure[16].
Figure 1. XRD patterns and Raman spectra of the sintered ceramics. (A) XRD patterns from 20° to 75°; (B) expanded XRD patterns at around peak (111); (C) Raman spectra in the range of 100-900 cm-1.
The Raman spectra of (1-x)BNBT-xSBTZ ceramics were analyzed in the range of 100-1,000 cm-1, as depicted in Figure 1C. The spectra were categorized into four regions, each representing distinct chemical bonding behavior[17]. The modes below 200 cm-1 were found to be associated with A-site atomic vibrations, while the high-frequency band between 200 and 380 cm-1 corresponded to the vibrations of BO bonds, including Zr2+, Zr4+, and Ti4+ ions. The modes near 550 cm-1 and 610 cm-1 were linked to Ti/Zr-O6 vibrations, and it was observed that the two peak positions became separated, indicating an increased intensity of oxygen octahedron vibration and enhanced relaxation characteristics with increasing doping content[18]. The section with modes above 700 cm-1 was considered to be the superposition of various vibration modes.
The SEM images of (1-x)BNBT-xSBTZ ceramics, as depicted in Figure 2A-D, indicate that the ceramics have been effectively sintered with minimal surface pores. The grain size of the ceramics initially decreases and then increases with increasing dopant concentration. This phenomenon can be attributed to the appearance of low melting point regions in the ceramics after SBTZ doping. During the sintering and cooling process, these regions are more prone to nucleation and grain growth, resulting in a decrease in grain size. However, at higher doping concentrations, the liquid phase melting regions further increase, leading to the emergence of smaller grain sizes[19]. The breakdown field of ceramics is significantly influenced by the grain size, with smaller grain sizes resulting in higher breakdown electric fields[18]. Therefore, the addition of SBTZ at low doping levels is advantageous for achieving high energy storage properties. Specifically, when the SBTZ content is 0.07, 0.08, 0.09, and 0.1, the average grain sizes are
Figure 2. The surface morphology of the (1-x)BNBT-xSBTZ ceramics and distribution of grain size. (A) x = 0.07, (B) x = 0.09,
Figure 3 depicts the dielectric behavior of (1-x)BNBT-xSBTZ ceramics over a temperature range of
Figure 3. Temperature-dependent dielectric permittivity and loss of (1-x)BNBT-xSBTZ ceramics from 70 to 400 °C at 1 kHz, 10 kHz,
In order to conduct a more thorough investigation into the relaxor transition in ceramics, an adaptation of the Curie-Weiss Law was implemented, which can be expressed as follows:
where ε is the dielectric constant, T is the temperature, Tm is the temperature at which ε reaches its maximum value εmax, C is a constant, and is the diffuse degree. The value of γ ranges from 1 for typical ferroelectrics to 2 for ideal relaxor ferroelectrics.
Figure 4 depicts the relationship between ln(T-Tm) and ln(1/ε-1/εmax) for the examined samples. The calculated value of γ ranges from 1.76 to 2.01, indicating a strong relaxation behavior of the sintered ceramics.
Figure 4. Plots of ln(1/ε-1/εmax) as a function of ln(T-Tm) at 1 kHz by the modified Curie-Weiss law.
The dielectric breakdown strength (DBS) of the ceramics was evaluated, as depicted in Figure 5A. By utilizing the Weibull distribution as Equation (5-7)[22]:
Figure 5. Energy storage studies of sintered (1-x)BNBT-xSBTZ ceramics. (A) Weibull distribution of the breakdown electric field; (B) P-E loops as a function of x; (C) Pm, Pr, and Pm-Pr as a function of x; (D) Wr, W, and η as a function of x.
where i is the group number, Xi and Yi are parameters in Weibull distribution functions, Ei is the DBS of sample i, Pi is associated with dielectric breakdown, and n is the sum of specimens of each sample. The electric field is ranged as Equation (8).
According to the Weibull distribution model, there is a linear relationship between Xi and Yi, as shown in Equation (9).
where m represents the dispersion degree of all experimental data, and a larger m indicates more reliable experimental results.
The determination of the magnitude of DBS can be derived from the intercept on the ln(Ei) axis. The results obtained from the fitted calculations in Figure 5A indicate that the DBS gradually increases and then decreases with the increase of SBTZ content, which is consistent with the shift in the average grain size. The DBS values obtained for increasing dopant concentrations were 215 kV/cm, 230 kV/cm, 245 kV/cm, and 218 kV/cm, respectively. The DBS is influenced by several factors, including porosity, grain size, and extrinsic measurement conditions such as sample thickness, sample area, and electrode configuration. In this study, all the samples had almost the same phase structure and measurement condition, but there was a slight difference in grain size. Generally, smaller grain size contributes to an enhanced DBS, and furthermore, the DBS exhibits an opposite variation tendency with grain size[23-25].
The polarization hysteresis (P-E) loops of the sintered BNBT-xSBTZ ceramics were measured and shown in Figure 5B. The test electric field was loaded near the breakdown field. The P-E loops were observed to be flat and slender, indicating weak ferroelectricity and enhanced relaxation properties. This is a crucial factor in achieving high energy storage density and efficiency[26]. As the doping content increased, Pmax, Pr, and Pmax-Pr gradually decreased, as depicted in Figure 5C. The trend of decreasing polarization intensity also suggests a decline in ferroelectricity. The sample with 0.93BNBT-0.07SBTZ exhibited the highest Pmax of 44.38 μC/cm2, but its Wr was not the highest due to its high Pr. In contrast, the 0.9BNBT-0.1SBTZ sample had a lower Pmax of 29.04 μC/cm2 and Pr of 2.36 μC/cm2. Figure 5D illustrates the values of W, Wr, and η for all samples. It was observed that Wr reached its maximum value at x=0.09, while η was highest at x=0.10. The values of W, Wr, and η are presented in Table 1.
Energy storage properties of (1-x)BNBT-xSBTZ ceramics
| x | W (J/cm3) | Wr (J/cm3) | E (kV/cm) | η |
| 0 | 1.38 | 0.174 | 60 | 12.6% |
| 0.07 | 3.02 | 1.91 | 215 | 63.1% |
| 0.08 | 3.21 | 2.37 | 230 | 73.9% |
| 0.09 | 3.60 | 2.68 | 245 | 74.4% |
| 0.10 | 2.84 | 2.38 | 218 | 83.9% |
The practical application of energy storage ceramics requires good temperature stability. To investigate the temperature stability of the 0.91BNBT-0.09SBTZ ceramic, energy storage characteristics were examined at various temperatures. P-E hysteresis loops were measured at 170 kV/cm, with temperatures ranging from
Figure 6. Temperature stability and fatigue resistance of the sintered ceramics. (A) P-E hysteresis loops at 25 °C to 140 °C; (B) Wr and η as a function of temperature; (C) fatigue behavior measured at various electrical cycles; (D) Wr and η as a function of electrical cycles.
In addition to examining the temperature stability of ceramics, their fatigue resistance behavior was also investigated. The fatigue behavior was assessed at room temperature under a frequency of 10 Hz and a voltage of 170 kV/cm, with a cycling number of 105. The P-E loops at different cycles were depicted in Figure 6C, while the changes in Pmax and Wr with an increase in the number of charging/discharging cycles were illustrated in Figure 6D. After 105 cycles, Wr remained at a relatively high level of 1.5 J/cm3, which was only 13.3% lower than its initial value of 1.73 J/cm3, indicating favorable fatigue resistance properties. It is worth noting that Pmax and Wr exhibited a slight decrease, which was attributed to the local phase decomposition caused by switching-induced charge injection, as suggested by Lou et al.[27-29].
CONCLUSIONS
This study investigated the dielectric and energy storage properties of (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)
DECLARATIONS
Authors’ contributionsMade substantial contributions to the conception and design of the study and performed data analysis and interpretation: Wang C
Performed data acquisition and provided administrative, technical, and material support: Lou X
Availability of data and materialsNot applicable.
Financial support and sponsorshipThis work was supported by the Education Science and Technology Innovation Project of Gansu Province, China, the year of 2022; the project: Study on the mechanism of the fatigue behavior of sodium-bismuth-titanate based relaxor ferroelectric capacitors after electrical charge-discharge cycles (Grant No. 2022QB-213) and the National Natural Science Foundation of China (NSFC NO.52172125).
Financial support and sponsorshipNone.
Conflicts of interestBoth authors declared that there are no conflicts of interest.
Ethical approval and consent to participateNot applicable.
Consent for publicationNot applicable.
Copyright© The Author(s) 2023.
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Wang C, Lou X. High energy storage properties of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics by incorporating
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